J. Watermann - Academia.edu (original) (raw)
Papers by J. Watermann
Astrophysics and Space Science Library, 2007
Journal of Space Weather and Space Climate, 2017
Advances in Space Research, 2006
To make deterministic medium-range (1 to 3 days ahead) forecasts would require prediction of the ... more To make deterministic medium-range (1 to 3 days ahead) forecasts would require prediction of the detailed near-Earth solar-wind conditions-including the out-of-ecliptic interplanetary magnetic field component-days ahead from remote observations of the inner heliosphere. This is, however, not feasible and will not be in the foreseeable future. Nevertheless, important steps can be taken towards a more physics based approach to geomagnetic forecasting. In this report from an ongoing study within the ESA Space Weather Applications Pilot Project, we discuss presently used concepts and new ideas for fully automatic provision of geomagnetic activity forecasts hours to days ahead. Observations of eruptive events on the Sun are nowadays routinely used as predictors of strong (Dst < À100 nT) magnetic storms, but these predictions are still hampered by many false alarms. We here suggest to use the flux of >10 MeV solar energetic particles as a discriminator of potentially geoeffective halo coronal mass ejections, substantially reducing the false alarm rate. We also describe how low speed/high speed solar-wind stream interfaces can be automatically detected in currently available solar-wind models, and point out that, in the absence of eruptive events on the Sun, these may be used as predictors of moderate (Dst < À50 nT) magnetic storms, effectively introducing a deterministic element into the mainly probabilistic forecast schemes.
Technische Informationsbibliothek Hannover: RN 2598 (81-111)+a-b / FIZ - Fachinformationszzentrum... more Technische Informationsbibliothek Hannover: RN 2598 (81-111)+a-b / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman
Annals of Geophysics, 1998
The alternating magnetic dynamo field of sea surface waves, a consequence of their Lorentz electr... more The alternating magnetic dynamo field of sea surface waves, a consequence of their Lorentz electric field, has been observed with a pair of simultaneously operated, closely spaced tri-axial magnetometers. Measurements from a magnetometer located in the centre of a tiny, uninhabited island served to compensate measurements from a near-shore magnetometer for magnetic pulsations of ionospheric origin, leaving the ocean wave dynamo field, effective close to shore only, as the dominant residual magnetic field. Amplitude and frequency of waves and swell were recorded with a vertical accelerometer (wave rider buoy) floating nearby on the sea surface. A spectral analysis was performed on ten nighttime intervals of three hours length each, and for every interval, the peak power of the surface waves (obtained from the wave rider) was compared with the peak power of the residual horizontal magnetic field (after the background field had been removed). The results suggest that the dual-sensor ma...
Journal of Space Weather and Space Climate, 2019
The three-dimensional configuration of the four Cluster satellites can be used, in conjunction wi... more The three-dimensional configuration of the four Cluster satellites can be used, in conjunction with ground observations, to analyze both the propagation and the coherence scale of ULF waves in the outer magnetosphere. We report here on the results of analysis of several events in late 2001 in the Pc 3-4 band (15-100 mHz), both on closed dayside field lines and in the polar cap. We have found poor correlations between transverse wave components at the 4 Cluster spacecraft when they were in the dayside middle magnetosphere, with separation distances of ~1000 - 5000 km. Correlation was considerably better for the field-aligned component, suggesting that compressional waves correlated well but that the transverse wave scale (e.g., of local field line resonances) was quite short. In contrast, all components correlated much better for the same range of separation distances when the satellites were in the polar cap. We interpret this as showing the lack of local resonances, and thus the do...
Comptes Rendus Geoscience, 2010
The Sun provides the main energy input to the terrestrial atmosphere, and yet the impact of solar... more The Sun provides the main energy input to the terrestrial atmosphere, and yet the impact of solar variability on long-term changes remains a controversial issue. Direct radiative forcing is the most studied mechanism. Other much weaker mechanisms, however, can have a significant leverage, but the underlying physics is often poorly known. We review the main mechanisms by which solar variability may impact the terrestrial atmosphere, on time scales ranging from days to millennia. This includes radiative forcing, but also the effect of interplanetary perturbations and energetic particle fluxes, all of which are eventually driven by the solar magnetic field. slightly expanded version of an article to appear in Comptes Rendus Geoscience (2009), special issue on The atmosphere observed from space.
Journal of Geophysical Research: Space Physics, 2008
We discuss the motion and structure of the magnetopause/boundary layer observed by Cluster in res... more We discuss the motion and structure of the magnetopause/boundary layer observed by Cluster in response to a joint tangential discontinuity/vortex sheet (TD/VS) observed by the Advanced Composition Explorer spacecraft on 7 December 2000. The observations are then supplemented by theory. Sharp polarity reversals in the east-west components of the field and flow B y and V y occurred at the discontinuity. These rotations were followed by a period of strongly northward interplanetary magnetic field (IMF). These two factors elicited a two-stage response at the magnetopause, as observed by Cluster situated in the boundary layer at the duskside terminator. First, the magnetopause suffered a large deformation from its equilibrium position, with large-amplitude oscillations of 3−minperiodbeingsetup.ThesearearguedtobemainlytheresultoftangentialstressesassociatedwithDVythecontributionofdynamicpressurechangesbeingsmallincomparison.Thisstrengthensrecentevidenceoftheimportancetomagnetosphericdynamicsofchangesinazimuthalsolarwindflow.TheTD/VSimpactcausedaglobalresponseseenbygroundmagnetometersinamagneticlocaltimerangespanningatleast12h.Theresponsemonitoredongroundmagnetometersissimilartothatbroughtaboutbymagnetopausemotionsdrivenbydynamicpressurechanges.Second,Clusterrecordedhigher−frequencywaves(3-min period being set up. These are argued to be mainly the result of tangential stresses associated with DV y the contribution of dynamic pressure changes being small in comparison. This strengthens recent evidence of the importance to magnetospheric dynamics of changes in azimuthal solar wind flow. The TD/VS impact caused a global response seen by ground magnetometers in a magnetic local time range spanning at least 12 h. The response monitored on ground magnetometers is similar to that brought about by magnetopause motions driven by dynamic pressure changes. Second, Cluster recorded higher-frequency waves (3−minperiodbeingsetup.ThesearearguedtobemainlytheresultoftangentialstressesassociatedwithDVythecontributionofdynamicpressurechangesbeingsmallincomparison.Thisstrengthensrecentevidenceoftheimportancetomagnetosphericdynamicsofchangesinazimuthalsolarwindflow.TheTD/VSimpactcausedaglobalresponseseenbygroundmagnetometersinamagneticlocaltimerangespanningatleast12h.Theresponsemonitoredongroundmagnetometersissimilartothatbroughtaboutbymagnetopausemotionsdrivenbydynamicpressurechanges.Second,Clusterrecordedhigher−frequencywaves(79 s). Two clear phases could be distinguished from the spectral power density, which decreased by a factor of $3 in the second phase. Applying compressible linearized MHD theory, we show that these waves are generated by the Kelvin-Helmholtz (KH) instability. Varying the local magnetic shear at the Cluster locale, as suggested by the temporal profile of the IMF clock angle, we find that locally stability was reinstated, so that the reduced power in the second phase is argued to be due residual KH activity arriving from locations farther to the dayside.
The combination of Magsat and Ørsted high-precision magnetic measurements has provided detailed a... more The combination of Magsat and Ørsted high-precision magnetic measurements has provided detailed and consistent patterns of ionospheric field-aligned current (FAC) patterns during intervals of northward interplanetary magnetic field (NBZ) conditions. The NBZ FAC currents extend from two ionospheric regions centered at 85 deg geo- magnetic latitude and at magnetic local times of approximately 0900 for the upward and 1500 for the downward FAC. The total intensity of the upward and downward NBZ FAC's are about equal. The locations of the NBZ FAC regions are almost in- variant to seasonal variations and to variations in the magnitude of IMF Bz. The NBZ FAC currents are sizeable for summer conditions only and vanish during the winter seasons. The NBZ FAC intensity is a strongly non-linear function of IMF Bz with a marked threshold at around +5 nT. These observations support experiences from po- lar magnetometer observations, among other, from Greenland used to investigate the developm...
Using observations from the Greenland magnetometer chain at 1-Hz sampling rate we have the possib... more Using observations from the Greenland magnetometer chain at 1-Hz sampling rate we have the possibility to study ULF waves over a wide frequency band and a latitude range which extends across most of the auroral zone and deep into the polar cap, i.e. covering the transition from field lines which are often closed in the magnetosphere to those which are always open. We examine Greenland magnetometer observations from 2003, 2004 and 2005 with the objective to characterize the frequency and latitude dependence of ULF wave activity during quiet and stormy times. To do so we select several frequency bands and bin the magnetometer observations according to the level of geomagnetic activity as defined by various geomagnetic indices. We further infer statistical patterns of ULF activity between 66 and 85 deg geomagnetic latitude over all magnetic local times. The investigation includes examining the temporal development of ULF wave activity in relation to the progress of a magnetic storm.
We have studied more than 50 cases of cusp observations from low-altitude satellites during the F... more We have studied more than 50 cases of cusp observations from low-altitude satellites during the February 16-22, 2002, SIRCUS campaign period. About half of them were inferred from DMSP F-13/14/15 particle spectrometer data and the other half from magnetic field measurements in the lower ELF range made onboard the CHAMP and Ørsted LEO satellites. The locations of the satellites during the detection of these signatures were converted into AACGM coordinates, and the geomagnetic latitude of these signatures set in relation to the statistical, IMF-Bz dependent particle cusp latitude derived by Newell et al. (1989). The particle cusp latitude which we inferred from DMSP data matches the statistically expected latitude while the small-scale magnetic field regime appears to cover the equatorward section of the cusp and the poleward section of the low-latitude boundary layer (LLBL). We suggest that the perturbations resulting in small-scale magnetic field variations are generated in the LLBL...
We have studied more than 50 cases of cusp crossings by low-altitude satellites during the Februa... more We have studied more than 50 cases of cusp crossings by low-altitude satellites during the February 16-22 2002 SIRCUS campaign period About half of them were inferred from DMSP F-13 14 15 particle spectrometer observations and the other half from measurements of small-scale magnetic field variations made onboard the Orsted and CHAMP satellites The locations of the satellites during the detection of the respective cusp signatures were converted into AACGM coordinates and the geomagnetic latitude of these signatures set in relation to the statistical IMF-Bz dependent particle cusp latitude derived by Newell et al 1989 The observed particle cusp latitude inferred from DMSP measurements matches the statistically expected cusp latitude well while the small-scale magnetic field perturbation regime appears to cover not only the cusp but also the poleward section of the low-latitude boundary layer LLBL close to noon We suggest that the perturbations resulting in the small-scale magnetic fie...
Astrophysics and Space Science Library, 2007
Journal of Space Weather and Space Climate, 2017
Advances in Space Research, 2006
To make deterministic medium-range (1 to 3 days ahead) forecasts would require prediction of the ... more To make deterministic medium-range (1 to 3 days ahead) forecasts would require prediction of the detailed near-Earth solar-wind conditions-including the out-of-ecliptic interplanetary magnetic field component-days ahead from remote observations of the inner heliosphere. This is, however, not feasible and will not be in the foreseeable future. Nevertheless, important steps can be taken towards a more physics based approach to geomagnetic forecasting. In this report from an ongoing study within the ESA Space Weather Applications Pilot Project, we discuss presently used concepts and new ideas for fully automatic provision of geomagnetic activity forecasts hours to days ahead. Observations of eruptive events on the Sun are nowadays routinely used as predictors of strong (Dst < À100 nT) magnetic storms, but these predictions are still hampered by many false alarms. We here suggest to use the flux of >10 MeV solar energetic particles as a discriminator of potentially geoeffective halo coronal mass ejections, substantially reducing the false alarm rate. We also describe how low speed/high speed solar-wind stream interfaces can be automatically detected in currently available solar-wind models, and point out that, in the absence of eruptive events on the Sun, these may be used as predictors of moderate (Dst < À50 nT) magnetic storms, effectively introducing a deterministic element into the mainly probabilistic forecast schemes.
Technische Informationsbibliothek Hannover: RN 2598 (81-111)+a-b / FIZ - Fachinformationszzentrum... more Technische Informationsbibliothek Hannover: RN 2598 (81-111)+a-b / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman
Annals of Geophysics, 1998
The alternating magnetic dynamo field of sea surface waves, a consequence of their Lorentz electr... more The alternating magnetic dynamo field of sea surface waves, a consequence of their Lorentz electric field, has been observed with a pair of simultaneously operated, closely spaced tri-axial magnetometers. Measurements from a magnetometer located in the centre of a tiny, uninhabited island served to compensate measurements from a near-shore magnetometer for magnetic pulsations of ionospheric origin, leaving the ocean wave dynamo field, effective close to shore only, as the dominant residual magnetic field. Amplitude and frequency of waves and swell were recorded with a vertical accelerometer (wave rider buoy) floating nearby on the sea surface. A spectral analysis was performed on ten nighttime intervals of three hours length each, and for every interval, the peak power of the surface waves (obtained from the wave rider) was compared with the peak power of the residual horizontal magnetic field (after the background field had been removed). The results suggest that the dual-sensor ma...
Journal of Space Weather and Space Climate, 2019
The three-dimensional configuration of the four Cluster satellites can be used, in conjunction wi... more The three-dimensional configuration of the four Cluster satellites can be used, in conjunction with ground observations, to analyze both the propagation and the coherence scale of ULF waves in the outer magnetosphere. We report here on the results of analysis of several events in late 2001 in the Pc 3-4 band (15-100 mHz), both on closed dayside field lines and in the polar cap. We have found poor correlations between transverse wave components at the 4 Cluster spacecraft when they were in the dayside middle magnetosphere, with separation distances of ~1000 - 5000 km. Correlation was considerably better for the field-aligned component, suggesting that compressional waves correlated well but that the transverse wave scale (e.g., of local field line resonances) was quite short. In contrast, all components correlated much better for the same range of separation distances when the satellites were in the polar cap. We interpret this as showing the lack of local resonances, and thus the do...
Comptes Rendus Geoscience, 2010
The Sun provides the main energy input to the terrestrial atmosphere, and yet the impact of solar... more The Sun provides the main energy input to the terrestrial atmosphere, and yet the impact of solar variability on long-term changes remains a controversial issue. Direct radiative forcing is the most studied mechanism. Other much weaker mechanisms, however, can have a significant leverage, but the underlying physics is often poorly known. We review the main mechanisms by which solar variability may impact the terrestrial atmosphere, on time scales ranging from days to millennia. This includes radiative forcing, but also the effect of interplanetary perturbations and energetic particle fluxes, all of which are eventually driven by the solar magnetic field. slightly expanded version of an article to appear in Comptes Rendus Geoscience (2009), special issue on The atmosphere observed from space.
Journal of Geophysical Research: Space Physics, 2008
We discuss the motion and structure of the magnetopause/boundary layer observed by Cluster in res... more We discuss the motion and structure of the magnetopause/boundary layer observed by Cluster in response to a joint tangential discontinuity/vortex sheet (TD/VS) observed by the Advanced Composition Explorer spacecraft on 7 December 2000. The observations are then supplemented by theory. Sharp polarity reversals in the east-west components of the field and flow B y and V y occurred at the discontinuity. These rotations were followed by a period of strongly northward interplanetary magnetic field (IMF). These two factors elicited a two-stage response at the magnetopause, as observed by Cluster situated in the boundary layer at the duskside terminator. First, the magnetopause suffered a large deformation from its equilibrium position, with large-amplitude oscillations of 3−minperiodbeingsetup.ThesearearguedtobemainlytheresultoftangentialstressesassociatedwithDVythecontributionofdynamicpressurechangesbeingsmallincomparison.Thisstrengthensrecentevidenceoftheimportancetomagnetosphericdynamicsofchangesinazimuthalsolarwindflow.TheTD/VSimpactcausedaglobalresponseseenbygroundmagnetometersinamagneticlocaltimerangespanningatleast12h.Theresponsemonitoredongroundmagnetometersissimilartothatbroughtaboutbymagnetopausemotionsdrivenbydynamicpressurechanges.Second,Clusterrecordedhigher−frequencywaves(3-min period being set up. These are argued to be mainly the result of tangential stresses associated with DV y the contribution of dynamic pressure changes being small in comparison. This strengthens recent evidence of the importance to magnetospheric dynamics of changes in azimuthal solar wind flow. The TD/VS impact caused a global response seen by ground magnetometers in a magnetic local time range spanning at least 12 h. The response monitored on ground magnetometers is similar to that brought about by magnetopause motions driven by dynamic pressure changes. Second, Cluster recorded higher-frequency waves (3−minperiodbeingsetup.ThesearearguedtobemainlytheresultoftangentialstressesassociatedwithDVythecontributionofdynamicpressurechangesbeingsmallincomparison.Thisstrengthensrecentevidenceoftheimportancetomagnetosphericdynamicsofchangesinazimuthalsolarwindflow.TheTD/VSimpactcausedaglobalresponseseenbygroundmagnetometersinamagneticlocaltimerangespanningatleast12h.Theresponsemonitoredongroundmagnetometersissimilartothatbroughtaboutbymagnetopausemotionsdrivenbydynamicpressurechanges.Second,Clusterrecordedhigher−frequencywaves(79 s). Two clear phases could be distinguished from the spectral power density, which decreased by a factor of $3 in the second phase. Applying compressible linearized MHD theory, we show that these waves are generated by the Kelvin-Helmholtz (KH) instability. Varying the local magnetic shear at the Cluster locale, as suggested by the temporal profile of the IMF clock angle, we find that locally stability was reinstated, so that the reduced power in the second phase is argued to be due residual KH activity arriving from locations farther to the dayside.
The combination of Magsat and Ørsted high-precision magnetic measurements has provided detailed a... more The combination of Magsat and Ørsted high-precision magnetic measurements has provided detailed and consistent patterns of ionospheric field-aligned current (FAC) patterns during intervals of northward interplanetary magnetic field (NBZ) conditions. The NBZ FAC currents extend from two ionospheric regions centered at 85 deg geo- magnetic latitude and at magnetic local times of approximately 0900 for the upward and 1500 for the downward FAC. The total intensity of the upward and downward NBZ FAC's are about equal. The locations of the NBZ FAC regions are almost in- variant to seasonal variations and to variations in the magnitude of IMF Bz. The NBZ FAC currents are sizeable for summer conditions only and vanish during the winter seasons. The NBZ FAC intensity is a strongly non-linear function of IMF Bz with a marked threshold at around +5 nT. These observations support experiences from po- lar magnetometer observations, among other, from Greenland used to investigate the developm...
Using observations from the Greenland magnetometer chain at 1-Hz sampling rate we have the possib... more Using observations from the Greenland magnetometer chain at 1-Hz sampling rate we have the possibility to study ULF waves over a wide frequency band and a latitude range which extends across most of the auroral zone and deep into the polar cap, i.e. covering the transition from field lines which are often closed in the magnetosphere to those which are always open. We examine Greenland magnetometer observations from 2003, 2004 and 2005 with the objective to characterize the frequency and latitude dependence of ULF wave activity during quiet and stormy times. To do so we select several frequency bands and bin the magnetometer observations according to the level of geomagnetic activity as defined by various geomagnetic indices. We further infer statistical patterns of ULF activity between 66 and 85 deg geomagnetic latitude over all magnetic local times. The investigation includes examining the temporal development of ULF wave activity in relation to the progress of a magnetic storm.
We have studied more than 50 cases of cusp observations from low-altitude satellites during the F... more We have studied more than 50 cases of cusp observations from low-altitude satellites during the February 16-22, 2002, SIRCUS campaign period. About half of them were inferred from DMSP F-13/14/15 particle spectrometer data and the other half from magnetic field measurements in the lower ELF range made onboard the CHAMP and Ørsted LEO satellites. The locations of the satellites during the detection of these signatures were converted into AACGM coordinates, and the geomagnetic latitude of these signatures set in relation to the statistical, IMF-Bz dependent particle cusp latitude derived by Newell et al. (1989). The particle cusp latitude which we inferred from DMSP data matches the statistically expected latitude while the small-scale magnetic field regime appears to cover the equatorward section of the cusp and the poleward section of the low-latitude boundary layer (LLBL). We suggest that the perturbations resulting in small-scale magnetic field variations are generated in the LLBL...
We have studied more than 50 cases of cusp crossings by low-altitude satellites during the Februa... more We have studied more than 50 cases of cusp crossings by low-altitude satellites during the February 16-22 2002 SIRCUS campaign period About half of them were inferred from DMSP F-13 14 15 particle spectrometer observations and the other half from measurements of small-scale magnetic field variations made onboard the Orsted and CHAMP satellites The locations of the satellites during the detection of the respective cusp signatures were converted into AACGM coordinates and the geomagnetic latitude of these signatures set in relation to the statistical IMF-Bz dependent particle cusp latitude derived by Newell et al 1989 The observed particle cusp latitude inferred from DMSP measurements matches the statistically expected cusp latitude well while the small-scale magnetic field perturbation regime appears to cover not only the cusp but also the poleward section of the low-latitude boundary layer LLBL close to noon We suggest that the perturbations resulting in the small-scale magnetic fie...